Someone once explained to me that autistic children live their lives in boxes. Home things belong in the home box and school things belong in the school box, so problems start when you try to put school things(ie homework) in the home box.
Another problem is that autistic children sometimes find it difficult to transfer skills, so they are able to something in the school environment, because they have been taught how to do it there, but are completely unable to do the same thing in a different environment.
Our son had all these problems and eventually his school arranged for him to do his homework at lunchtime in the Homework club run by the support assistants. This was a reasonable adjustment.
I would not be spending 6 hours a week on homework. Apart from anything else your poor daughter is probably exhausted from coping with the new school and the school day, and she can't cope with any more work when she gets home. This is why you are having all the meltdowns - she has reached the end of her tether.
I would make it very clear to the school that your daughter is struggling and is extremely stressed out by the demands being made of her. I would also remind them very firmly that your daughter has a disability and is covered by the Equality Act 2010.
Under the Equality Act, the school has a legal duty to make "reasonable adjustments" for students with a disability, so they either exempt her from homework, allow her to do it in school at lunchtime (with support) or come up with some other plan, but she will not be doing it at home.
The Exit pass you spoke of upthread would also be a reasonable adjustment. Not to make reasonable adjustments can be construed as disability discrimination.
www.autism.org.uk/about/in-education/resolving-disagreements/discrimination-gb.aspx
www.ipsea.org.uk/types-of-disability-discrimination